Suspension Science: How Do Bridge Designs Compare?

Building a Better Bridge: Learn how engineers make bridges of different strengths using simple cables. And get ready to count your pennies as you test how much weight your bridge can hold.
George Retseck

Key concepts
Bridges
Forces
Load
Engineering

Introduction
Have you ever ridden in a car driving across a suspension bridge? Suspension bridges, with their tall towers, long spans and gracefully curving cables, are beautiful examples of the work of civil engineers. How do the cables and towers withstand the load of the bridge—including you and the car you're in? Can a suspension bridge carry a greater load than a simple beam bridge that doesn't use cables? You can try to answer these questions in this suspenseful science activity!

Background
A beam bridge is the simplest type of bridge. It is typically supported by a raised part on either end. For example, a beam bridge could be as simple as a wood plank put down to cross a stream.

Suspension bridges are a bit more complicated. They comprise a deck (the long straight part that includes the road), cables and towers. The cables stretch between the towers and help support the weight of the deck and its load. In this type of bridge the cables are under tension and the towers undergo compression.

Suspension bridges might seem complicated, but for spanning long distances they can also be the most economical—they require less material per foot than would a simpler beam bridge. Because suspension bridges are relatively flexible, however, high winds and other forces can be a serious problem. The dramatic collapse in 1940 of the Tacoma Narrows Bridge in Washington State is an infamous example of this. (You can watch a video of the disaster here.)

In this activity you'll build and test two types of bridges: a basic suspension bridge and a beam bridge. Which type of bridge do you think can support a heavier load?

Materials

• Seven disposable plastic drinking straws

• Masking tape or painter's tape

• Thread

• Scissors

• Four paper clips (At least two should be large ones.)

• Paper cup, at least eight ounces

• Many coins, all the same type (For example, you should have at least 150 quarters or at least 325 pennies.)

• Two chairs, tables or desks that can be arranged to build a bridge between, and on which tape can be used

Preparation

• If your straws are the bendable type, cut the flexible part off (so that you are left with a long, straight, nonbendable straw piece). Cut a total of six straws this way. Make sure they are all the same length; trim some if necessary. (If you are using nonflexible straws, use the whole straws for the long pieces.)

• Cut the seventh straw to make two short pieces of straw, each about one inch long. Make sure they are both the same length. (If it has a bendable part, cut your short pieces from the other end.)

• Tape two of the long straws to each end of one of the short pieces of straw.

• Then, tape the long straws together at their other ends. You should have an elongated triangle shape. This is one of the towers for your suspension bridge.

• Repeat this process with two new long straws and the other short straw piece to build the second tower.

• Tape one tower to the edge of a desk, table or chair. The short straw piece should be at the bottom of the tower (and the pointy end without a short piece should be at the top). Tape the second tower to a second piece of furniture at the same height. Position the towers far enough apart so that you could fit the remaining straws between them.

Procedure

• Place a long straw between the towers so that its ends rest on the short pieces. This straw is the deck. You now have a simple beam bridge. Can you see how this is a beam bridge? How do you think it would be different from a suspension bridge?

• Make a load tester for your bridge by unbending a large paperclip into a V-shape. Poke the ends of the paperclip into opposite sides of a paper cup, just below the thick rim at the top.

• Use a second large paperclip to hang the load tester over the bridge deck. Do this by attaching the two large paperclips together, and then sliding the new one around the bridge deck straw. Slide the cup to the middle of the straw deck.

• Add coins (all of the same type) one at a time into the load tester. Keep a count of how many coins you are adding. How many coins does the cup hold before the bridge fails? How does the bridge fail?

• Begin turning your structure into a suspension bridge. Pick a new long straw to serve as the deck.

• Cut a piece of thread about three feet long. Tie the center of the piece of thread (which will act as your bridge cable) around the middle of the new bridge deck straw. Place the straw between the towers as before.

• Pass each end of the thread over a tower and down the other side. To anchor the suspension bridge, tie each end of the cable around a paperclip. Slide the paperclips away from the towers until the cable pulls tight. Then tape the paperclips firmly to the furniture.

• Attach the empty load tester cup as you did before. Again add coins (all of the same type as before) to the cup, one at a time. How many coins does the cup hold before the bridge fails this time? How does the bridge fail?

• Overall, which bridge design is stronger? Is it a little stronger or a lot stronger? Why do you think you got the results that you did?

• Extra: Test each bridge design a few more times, using a new bridge deck straw each time. Are your results always the same?

• Extra: Try eliminating the portion of the cable from the towers to the anchorage (leaving only the portion of the cable from the bridge deck to the towers). What happens when you test your bridge with a load tester now? Why?

• Extra: Try this activity a few more times and focus on what part (or parts) of the bridge fails first. Was the failure due to weakness of materials used or weakness at a joint? Can you think of ways to redesign your bridge to make the part (or parts) that failed stronger?